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Old 12-08-2012, 12:02 PM
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Alt Light

The vehicle is a 1976 GMC Sprint, 350

I start the engine and it runs like a top. I rev it up to about 2,000 RPM and the Alternator light comes on. I thought it was the alternator so I put a volt meter at the battery with the car off. Voltage reading is 12.35. Start the engine and the voltage is 12.28. Engine running no Alt light. Rev it up the voltage jumps to 13.84 AND that is when the light comes on. As long as the alternator is charging the light is on. When I disconnect the two lead plug going to the alternator the voltage drops to 12.8 but the light is out. Reconnect the connector and the voltage stays the same until I rev the engine. Then the voltage goes up and the light comes back on. I have not run into this before so any help would be great.
Scholman

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Old 12-08-2012, 12:49 PM
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First Question...does your battery ever need an external charging? Also how many amps is the alternator putting out? Depending on the rating of your alternator, (some can be as high as 100 or 120 amps)?
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Old 12-08-2012, 01:16 PM
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69Widetrack,

No the battery has not needed a charge....ever
Not sure how many amps the alternator is putting out but it is rated at 65 amps. Just a stock unit.
I am thinking a wire has broken somewhere in the rats-nest near the distributor because this all started when I had to set the timing so I could get the car smogged
Scholman
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Old 12-08-2012, 01:35 PM
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Yes, check your wiring, you may have disturbed a ground wire. If you never needed to recharge your battery with a charger, than the system should be OK...It's 47 years old and who knows how many alterations have been made, let alone rust and corrosion on ground wires.

Ray
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Old 12-08-2012, 01:49 PM
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Quote:
Originally Posted by scholman View Post
The vehicle is a 1976 GMC Sprint, 350

I start the engine and it runs like a top. I rev it up to about 2,000 RPM and the Alternator light comes on. I thought it was the alternator so I put a volt meter at the battery with the car off. Voltage reading is 12.35. Start the engine and the voltage is 12.28. Engine running no Alt light. Rev it up the voltage jumps to 13.84 AND that is when the light comes on. As long as the alternator is charging the light is on. When I disconnect the two lead plug going to the alternator the voltage drops to 12.8 but the light is out. Reconnect the connector and the voltage stays the same until I rev the engine. Then the voltage goes up and the light comes back on. I have not run into this before so any help would be great.
Scholman
Check the two wire plug at the alternator. It sounds like the terminals are corroded or the wires bad causing you to have to rev the engine to get output.
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Old 12-08-2012, 02:02 PM
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loose wire

I have a case backhoe that uses a bosch alternator. It quit charging so i put in new brushes, very easy on the bosch. then it was sometimes charging and sometimes not. The case only runs a low amp draw fuel flow solenoid, once it is running it will go all day on an almost dead batery. I took it in to have it tested and it did the same thing on the test machine. the mechanic said it had a poor connection inside the alternator, he wanted 300 for a rebuild, $450 for a new bosch. So I just put the charger on every 4 or 5 days Until get time to put on a GM one wire
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Old 12-08-2012, 02:10 PM
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I had a similar deal w/a '85 Subaru parts chaser I owned a while back. The original alternator quit at 240K miles, they wanted an arm and a leg for a new one. Instead, I rewired the Subaru to use a GM alternator (SI type w/internal regulator). The mount was even close enough to work. Cost was nada, I already had the alternator left over from scrapping cars.
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Old 12-08-2012, 06:57 PM
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Yank the alternator off the car, take it down and have it tested, by a counterperson who knows the correct way to test it.
The voltage sounds good, so the regulator is OK.
Most likely problem will be a rectifier gone bad. That is why the volts look OK but the light glows dimly.
I bet it wont pass the current output (amps) test when checked.
I have had alternators that turn on the light when revved up, but werent charging when that happened, that would be the brushes in that instance, yours puts out 13.8 when revved, not going to be the brushes.
When a rectifier goes bad it usually goes open circuit and passes no current, on ocasion a rectifier will short out and pass alternating current. AC is pretty tough on batterys and electrical devices designed to run on DC.
The alternator makes alternating current, the rectifier is necessary to convert it to Direct current.
The rectifier is the aluminum block lookin finned thing in the pic below.
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Old 12-08-2012, 10:59 PM
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Would a bad rectifier cause it to not charge until the engine was revved up?
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Old 12-09-2012, 08:43 AM
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Yep. The 63-65 amp 10SI has a drop resistor in it that forces the alternator to "energize" at low RPMs to aid in charging under heavy loads at idle, and when started in case engine rpm arent fast enough to get the ball rolling.
Low output at slow enigne speed may keep the light out, but higher RPMs and regualtor demand will set it off as it cant produce enough current to keep up with the regulator demand.
Sounds like one of the 3 phases in the alternator is not outputting.
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